Exhaust gases discharged from diesel engines contain PM (particulate matter) mainly composed of SOFs (soluble organic fractions) comprising carbonaceous soot and high-boiling-point hydrocarbons, and the SOFs released into the air are likely to adversely affect human bodies and environment. Accordingly, PM-capturing ceramic honeycomb filters are conventionally mounted in exhaust pipes connected to diesel engines. One example of ceramic honeycomb filters for capturing PM to clean exhaust gases is shown in FIGS. 1(a) and 1(b). The ceramic honeycomb filter 10 comprises a ceramic honeycomb structure comprising porous cell walls 2 defining many outlet-side-sealed flow paths 3 and inlet-side-sealed flow paths 4 and a peripheral wall 1, and upstream-side plugs 6a and downstream-side plugs 6c for alternately sealing the inlet-side end surfaces 8 of the inlet-side-sealed flow paths 4 and the outlet-side end surfaces 9 of the outlet-side-sealed flow paths 3 in a checkerboard pattern.
As shown in FIG. 2, this ceramic honeycomb filter 10 is held in a metal container 12 with support members 14, and longitudinally sandwiched by support members 13a, 13b. The support members 14 are generally formed by a metal mesh and/or a ceramic mat. The ceramic honeycomb filter 10 attached to a diesel engine is subject to mechanical vibration and shock from the engine, the road, etc. via the support members 13a, 13b, 14. Particularly because large ceramic honeycomb filters of more than 200 mm in outer diameter for use in large vehicles or special vehicles are subject to large vibration and shock, they are required to keep enough strength.
Important characteristics among those required for ceramic honeycomb filters are particulate-matter-capturing efficiency, pressure loss, and particulate-matter-capturing time (a time period from the start of capturing to a point reaching a constant pressure loss). Particularly, the capturing efficiency and the pressure loss are in a contradictory relation; the higher capturing efficiency resulting in larger pressure loss and a shorter capturing time, and smaller pressure loss resulting in a longer capturing time and poorer capturing efficiency. To meet these contradictory filtering characteristics, investigation has conventionally been conducted to provide technologies for controlling the porosities, average pore sizes and pore sizes on the cell wall surfaces of ceramic honeycomb structures.
To meet further increased regulations of exhaust gases in recent years, exhaust-gas-cleaning apparatuses comprising both SCR apparatuses for removing NOx and honeycomb filters for removing particulate matter have been investigated, and honeycomb filters are required to have smaller pressure loss than conventional ones. Because ceramic honeycomb filters of more than 200 mm in outer diameter for use in large vehicles or special vehicles do not easily have enough strength to withstand mechanical vibration and shock during use, ceramic honeycomb filters having sufficient strength and pressure loss characteristics cannot be obtained by conventional technologies as shown below.
JP 61-129015 A discloses an exhaust-gas-cleaning filter having large pores having diameters of 40-100 μm, and small pores having diameters of 5-40 μm as many as 5-40 times the large pores on cell wall surfaces, this filter having high capturing efficiency from an early stage of use, and small pressure loss. It is further described that pores in cell walls preferably have an average pore size of more than 15 μm, and a cumulative pore volume of 0.3-0.7 cm3/g. The cumulative pore volume of 0.3-0.7 cm3/g is converted to porosity of 42.8-63.6% by volume. From a pore size distribution line shown in FIG. 4 of JP 61-129015 A, the honeycomb filters of Examples 1, 2, 5 and 6 have cumulative pore volumes of 0.58 cm3/g (porosity: 59%), 0.4 cm3/g (porosity: 50%), 0.7 cm3/g (porosity: 64%) and 0.3 cm3/g (porosity: 43%), respectively, and average pore sizes of 40 μm, 35 μm, 44 μm and 15 μm, respectively. The porosity P (% by volume) is determined from the true specific gravity p (2.5 g/cm3) and cumulative pore volume V (cm3/g) of cordierite, by the formula of P=100×V×ρ/(1+V×ρ).
However, particularly when used as large filters of more than 200 mm in outer diameter for large vehicles or special vehicles, the honeycomb filters of Examples 1, 2 and 5 of JP 61-129015 A do not have enough strength because of too large average pore sizes or porosity, and the honeycomb filter of Example 6 has insufficient pressure loss characteristics because of too small porosity. Namely, the honeycomb filters of Examples 1, 2, 5 and 6 do not have both small pressure loss and high strength.
JP 2002-219319 A discloses that porous honeycomb filters have high particulate-matter-capturing efficiency without pressure loss increase due to the clogging of pores, when their cell walls have such pores that the volume of pores having diameters of less than 10 μm is 15% or less, the volume of pores having diameters of 10-50 μm is 75% or more, and the volume of pores having diameters of more than 50 μm is 10% or less, based on the total pore volume. However, the pore structure described in JP 2002-219319 A fails to provide sufficient pressure loss characteristics and strength to large ceramic honeycomb filters of more than 200 mm in outer diameter for use in large vehicles or special vehicles.
JP 2004-322082 A discloses a ceramic honeycomb filter wherein the total pore volume is 0.55-0.80 cm3/g (corresponding to porosity of 59-67%), and the volume of pores of 100 μm or more is 0.02-0.10 cm3/g. The total pore volume range is 59-67%, when converted to porosity by the above formula. However, the pore structure described in JP 2004-322082 A need to be improved on pressure loss characteristics and strength to withstand mechanical vibration and shock during use, particularly for large ceramic honeycomb filters of more than 200 mm in outer diameter for large vehicles or special vehicles.
JP 2005-530616 A discloses a ceramic honeycomb filter, whose pore size distribution provides [d50/(d50+d90)] of less than 0.70, and Sf (=[d50/(d50+d90)]/[porosity (%)/100]) of less than 1.55, wherein Sf is a permeability factor when soot is attached to the filter. It describes that such pore size distribution provides smaller pressure loss. However, the pore structure described in JP 2005-530616 A is not satisfactory in both pressure loss characteristics and strength, particularly when used for large ceramic honeycomb filters of more than 200 mm in outer diameter for large vehicles or special vehicles.
JP 2007-525612 A discloses a diesel particulate matter filter having a median diameter d50 of less than 25 μm, and pore size distribution and porosity meeting the relation of Pm≦3.75, wherein Pm=10.2474 [1/[(d50)2(porosity (%)/100)]+0.0366183(d90)−0.00040119(d90)2+0.468815(100/porosity (%))2+0.0297715(d50)+1.61639(d50−d10)/d50], wherein d10 and d90 represent pore sizes at 10% and 90% of the total pore volume in a volume-based cumulative pore size distribution, d10<d50<d90. However, the pore structure described in JP 2007-525612 A does not have sufficient pressure loss characteristics and strength particularly when used for large ceramic honeycomb filters of more than 200 mm in outer diameter for large vehicles or special vehicles.
WO 2005/090263 A discloses a method for producing a porous ceramic body by forming a moldable ceramic material containing porous silica powder or porous silica-containing compound powder into a predetermined shape, and sintering it. It describes that porous silica powder or porous silica-containing compound powder functions as a pore-forming material for controlling the size and amount of pores. However, the silica particles described in WO2005/090263 A have a wide particle size distribution, with many coarse particles even when particles of the optimum median diameter are selected, forming cell walls having large pores. As a result, it does not have enough strength to withstand vibration and shock when mounted on vehicles.